I don't understand how the n for p2 and p3 is found when solving this problem. please go into detail with steps!

You are going to receive three perpetuities. Perpetuity 1 will pay $450 each year, with the first payment to be paid at Year 1. Perpetuity 2 will pay $650 each year, with the first payment to be paid at Year 15 . Perpetuity 3 will pay $750 each year, with the first payment to be paid at Year 31 . The effective annual interest rate is 8.58 percent. Find the combined value of these three perpetuities, evaluated at Year 25. Enter your answer to the nearest cent, with no punctuation other than a decimal. For example, if your answer is $28,542.19, enter "28542.19". Note that Canvas will delete trailing zeros, if entered